One could ask "why?" We know life begins at delivery. We don't know if it begins before, or how long before, and to conduct a thorough, scientific assessment would likely be inhumane.
I think an appropriately arbitrary figure would suffice if there is some overwhelming emotional need to define life as beginning before delivery. Like at the half way point, or two thirds point of being fully developed. Anything more "precise" than that is presumptive and unsupportable by facts and gets overly colored by emotions and religious beliefs.
It's a moving target...the earliest point at which prematurely delivered children stand a decent chance of survival is probably around 6-7 months, if not earlier. If you can remove a fetus from a womb and sustain it's life without causing permanent physical or mental impairment or even discomfort, sounds to me it's viable.
So I'll assume you believe in the "magic portal" version of the pro-choice squad, which is to say something mystical and inexplicable happens when the unborn child moves past that 1/4" or so of mom's tissue that separates it from the rest of the outside world, and until that happens, all bets are off.
Might take that up with any of the governors in states who think differently vis-a-vis prosecution of individuals who commit violent crimes against pregnant women - they make a pretty distinct accommodation for that case, not by accident.
My point is that where doubt exists, you have to give the fetus the benefit. You have to assume it would continue to gestate and grow to term and be delivered happy and healthy. Because there are several dozen billion instances of that happening throughout history - the vast, vast majority of pregnancies are carried to term without incident.